1. Field of the Invention
The present invention relates to devices which are known as contrast amplifiers and are employed for improving the visual perception of video images when these images have low contrast for such reasons as insufficient illumination, for example.
2. Description of the Prior Art
It is a known practice to enhance or amplify the contrast of video images by increasing the gain of the channel used for transmitting the luminance signal representing these images. This increase in gain is either uniform for all values of luminance or variable as a function of the luminance value. The disadvantage of this method is that it also increases the contrast of noise interference. Interference of this type becomes more troublesome as the contrast amplification is greater.
Another known method consists in equalizing the distribution of luminance values of each field throughout the range of possible values. To this end, the distribution of luminance values of a field is determined and each luminance value of the following field is corrected as a function of the distribution which is found in the case of the luminance values of the preceding field. French patent Application No. 2,456,448 filed by the present Applicant describes a method of this type. This known method has the advantage of putting the scale of luminance values to the best possible use but has the effect of increasing noise interference and is complex to apply in practice.
The contrast amplifier in accordance with the invention does not have the disadvantage of enhancing noise interference and is at the same time simple to construct. It is therefore particularly advantageous for processing images which are highly affected by noise. The amplifier makes advantageous use of the fact that noise affects only isolated points or a small number of interrelated points. The amplifier in accordance with the invention does not amplify the visibility of these points since it controls the gain of the channel which transmits the luminance signal, in a different manner at each image point, as a function of the luminance in the vicinity of this point and of the luminance of the entire image. The luminance in the vicinity of each point is determined by means of a so-called bidirectional spatial filtering process which consists of two recursive digital filtering operations performed successively. The mean luminance of an image can be estimated by considering the mean luminance of the preceding image in the case of a sequence of video images.